The present invention relates to a substrate potential generator for a semiconductor integrated circuit.
A substrate potential generator is used as a voltage supply to generate a prescribed electric voltage of a polarity opposite to that of an externally supplied power source voltage and to apply the prescribed electric voltage to a substrate of a semiconductor integrated circuit. Previously, as shown in FIG. 1, such a substrate potential generator is composed of a substrate potential detector 1 receiving a power supply potential V.sub.REF and a substrate potential V.sub.BB ' and a substrate potential supplier 2 for supplying the substrate potential to be controlled according to a substrate potential detection signal V.sub.D ' as an output from the substrate potential detector 1.
The operation of such a substrate potential generator is explained below. FIG. 2 shows a circuit diagram of an example of a substrate potential detector 1, wherein a P-type MOS transistor Q.sub.p21, an N-type MOS transistor Q.sub.n21 and an N-type MOS transistor Q.sub.n22 shorted between the gate and the drain thereof are connected in series. The gates of the transistors Q.sub.p21 and Q.sub.n21 are connected to the ground potential V.sub.SS. The source potential of the MOS transistor Q.sub.p21 is designated as supply potential V.sub.REF, while that of the MOS transistor Q.sub.n22 is equal to the substrate potential V.sub.BB ' received from the substrate potential supplier 2.
The source potential and the gate potential of the transistor Q.sub.p21 are equal to the supply potential V.sub.REF and the ground potential V.sub.BB ', respectively. The voltage between the gate and the source potentials is lower than the threshold voltage of the transistor Q.sub.p21, that is, the gate-to-source voltage is lower than the threshold voltage of the transistor Q.sub.p21, so that a drain current flows through the transistor Q.sub.p21. If the substrate potential V.sub.BB ' becomes lower than a set voltage which is lower than the ground potential V.sub.SS, the transistor Q.sub.n22 is turned on. Then, the potential at a connection point 26, that is, the source potential of the transistor Q.sub.n21 becomes lower than the threshold voltage of the transistor Q.sub.n21, so that the transistor Q.sub.n21 is also turned on. Therefore, because all the transistors Q.sub.p21, Q.sub.n21 and Q.sub.n22 are in the on-states, the drain potentials of the transistors Q.sub.p21 and Q.sub.n21 or the substrate potential detection signal V.sub.D ' becomes low enough to stop the operation of the substrate potential supplier 2.
On the contrary, if the substrate potential V.sub.BB ' floats up to a potential above the set value, the potential difference to lower the drain potential with the transistor Q.sub.n22 becomes small, so that the gate-to-source voltage of the transistor Q.sub.n21 is kept at a voltage lower or a little higher than its threshold voltage. Thus, the transistor Q.sub.n21 is turned off or only a small current can flow therethrough. Then, the substrate potential detection signal V.sub.D ' or the drain potentials of the transistors Q.sub.p21 and Q.sub.n21 increases until the drain current of the transistor Q.sub.n21 becomes equal to that of the transistor Q.sub.p21. Therefore, the substrate potential detection signal V.sub.D ' becomes a little lower than the source potential and high enough to activate the substrate potential supplier 2.
As explained above, the substrate potential detector 1 sends a low level substrate potential detection signal V.sub.D ' to the substrate potential in order to stop the action of the substrate potential supplier 2 if the actual substrate potential V.sub.BB ' is lower than the set potential on the basis of the supply potential and the substrate potential V.sub.BB, otherwise it sends a high-level substrate potential detection signal in order to activate the substrate potential supplier 2.
Further, the substrate potential supplier 2 is controlled according to the substrate potential detection signal V.sub.D '. If the substrate potential detection signal V.sub.D ' is a high-level, negative charges are supplied to the substrate until the substrate potential V.sub.BB ' becomes lower by the supply potential V.sub.REF than the threshold voltage of the transistor for supplying charges to the substrate. On the other hand, if the substrate potential detection signal V.sub.D ' is a low-level, the operation of the substrate potential supplier 2 is stopped so as not to supply negative charges to the substrate.
As explained above, the previous substrate potential generator can generate a high-level substrate potential detection signal V.sub.D ' according to the power supply potential V.sub.REF and the substrate potential V.sub.BB ' in the substrate potential detector 1 if the substrate potential V.sub.BB ' is higher than the set potential, so as to operate the substrate potential supplier 2 in order to lower the substrate potential V.sub.BB ' as long as the high level substrate potential detection signal V.sub.D ' is outputted. On the other hand, if the substrate potential V.sub.BB ' becomes lower below the set potential V.sub.set, the substrate potential detector 1 sends a low-level substrate potential detection signal V.sub.D ' to stop the operation of the substrate potential supplier 2 to make the substrate potential V.sub.BB ' equal to the set potential V.sub.set. If the substrate potential V.sub.BB ' becomes higher than the set potential V.sub.set again, the substrate potential detector 1 sends a high level substrate potential detection signal V.sub.D ' to activate the substrate potential supplier 2 in order to lower the substrate potential V.sub.BB ' again. These processes are repeated to make the substrate potential V.sub.BB ' the set potential V.sub.set.
However, in the previous substrate potential generator, the substrate potential detection signal V.sub.D ' for controlling the operation of the substrate potential supplier 2 is determined according to the set potential V.sub.set to be set at a point with respect to the substrate potential V.sub.BB '. Then, if the substrate potential V.sub.BB ' is around the set potential V.sub.set, the operation of the substrate potential supplier 2 is stopped if the substrate potential V.sub.BB ' becomes higher than the set potential V.sub.set, otherwise the operation is started again. Therefore, the number of stop and start repetition is high because the substrate potential supplier 2 is activated or stopped above or below the set potential V.sub.set at a point with respect to the substrate potential. Charge and discharge currents of the capacitances of signal lines and transistors are accompanied by the changes between the start and stop. Thus, a problem arises in that the current is enhanced even if the dissipation current of the substrate potential supplier 2 is decreased.